1. Field of Invention
The present invention relates to the field of downhole well tools. More specifically, the invention relates to a device and method for controlling downhole valves to obtain the desired flow characteristics through the valves.
2. Related Art
The economic climate of the petroleum industry demands that oil companies continually improve their recovery systems to produce oil and gas more efficiently and economically from sources that are continually more difficult to exploit and without increasing the cost to the consumer. One successful technique currently employed is the drilling of horizontal, deviated, and multilateral wells, in which a number of deviated wells are drilled from a main borehole. In such wells, as well as in standard vertical or near-vertical wells, the wellbore may pass through various hydrocarbon bearing zones or may extend through a single zone for a long distance. One manner to increase the production of the well is to perforate the well in a number of different locations, either in the same hydrocarbon bearing zone or in different hydrocarbon bearing zones, and thereby increase the flow of hydrocarbons into the well.
One problem associated with producing from a well in this manner relates to the control of the flow of fluids from the well and to the management of the reservoir. For example, in a well producing from a number of separate zones, or lateral branches in a multilateral well, in which one zone has a higher pressure than another zone, the higher pressure zone may produce into the lower pressure zone rather than to the surface. Similarly, in a horizontal well that extends through a single zone, perforations near the "heel" of the well--nearer the surface--may begin to produce water before those perforations near the "toe" of the well. The production of water near the heel reduces the overall production from the well. Likewise, gas coning may reduce the overall production from the well.
A manner of alleviating such problems may be to insert a production tubing into the well, isolate each of the perforations or lateral branches with packers, and control the flow of fluids into or through the tubing. However, typical flow control systems provide for either on or off flow control with no provision for throttling of the flow. To fully control the reservoir and flow as needed to alleviate the above-described problems, the flow must be throttled. A number of devices have been developed or suggested to provide this throttling although each has certain drawbacks. Note that throttling may also be desired in wells having a single perforated production zone.
Specifically, the prior devices are typically either wireline retrievable valves, such as those that are set within the side pocket of a mandrel, or tubing retrievable valves that are affixed to the tubing string. An example of a wireline retrievable valve is shown in U.S. patent application Ser. No. 08/912,150, by Ronald E. Pringle entitled "Variable Orifice Gas Lift Valve for High Flow Rates with Detachable Power Source and Method of Using Same" that was filed Aug. 15, 1997, and which is hereby incorporated herein by reference. The variable orifice valve shown in that application is selectively positionable in the offset bore of a side pocket mandrel and provides for variable flow control of fluids into the tubing.
A typical tubing retrievable valve is the standard "sliding sleeve" valve, although other types of valves such as ball valves, flapper valves, and the like may also be used. In a sliding sleeve valve, a sleeve having orifices radially therethrough is positioned in the tubing. The sleeve is movable between an open position, in which the sleeve orifices are aligned with orifices extending through the wall of the tubing to allow flow into the tubing, and a closed position, in which the orifices are not aligned and fluid cannot flow into the tubing.
Other types of downhole valves are numerous and include that shown in U.S. patent application Ser. No. 09/243,401, by David L. Malone, entitled "Valves for Use in Wells" that was filed Feb. 1, 1999, and U.S. patent application Ser. No. 09,325,474, entitled "Apparatus and Method for Controlling Fluid Flow in a Wellbore", by Ronald E. Pringle et al., that was filed Jun. 3, 1999, now U.S. Pat. No. 6,227,302 and which are hereby incorporated herein by reference. In general, the valve has valve covers that provide a seal around the periphery of the cover and the orifice through the tubing. The valve covers are sized in accordance with the size of the orifice. In this way, the surface of contact between the cover and the tubing, or seat, is much less than that encountered with a sliding sleeve and the stroke length is decreased. Additionally, the valve uses low coefficient of friction material, such as a polycrystalline diamond coating, to facilitate sliding and incorporates a self-cleaning feature aimed at removing built up debris that tends to impede valve movement.
Electric and hydraulic remote actuators for the downhole valves have been developed to overcome certain other difficulties often encountered with operating the valves in horizontal wells, highly deviated wells, and subsea wells using slickline or coiled tubing to actuate the valve. The remote actuators are positioned in the well proximal the valve to control the throttle position of the valve.
One problem associated with hydraulic type actuators relates to the limitations imposed by the incremental steps employed with such systems. Electrical actuators may be positioned at virtually any selected position between open and closed positions because of the flexibility of the motors. However, hydraulic actuators typically use indexing mechanisms to position the valves at incremental positions at and between open and closed positions. The number of increments between the open and closed positions may be relatively limited due to space and other limitations, with an example mechanism having six positions. In conventional indexing mechanisms, the distance between increments is generally equal. Further, the orifices provided in the valves are typically circular or otherwise generally uniform in shape. In one example arrangement, the first position is the closed position, the second incremental position opens the valve twenty percent, the third incremental position opens the valve forty percent, the fourth incremental position opens the valve sixty percent, the fifth incremental position opens the valve eighty percent, and the sixth position fully opens the valve.
The evenly spaced increments and the generally uniform hole sizes may raise several issues. The pressure drop across a valve is proportional to the inverse of the area squared. Therefore, a valve orifice in which the area at each increment varies generally linearly produces a change in the pressure drop and the flow rate that varies widely, with the pressure drop and flow rate change being greater when the valve is initially being opened as compared to the pressure drop and flow rate change due to the valve going from a nearly fully open position to the fully open position. For example, in the example mentioned above, the difference in the pressure drop (and flow rate) change between the twenty percent open and forty percent open conditions is much greater than that between the sixty percent open and eighty percent open conditions. One of the key design considerations and uses for a downhole valve is the control of the pressure drop and flow rate. Thus, there remains a need for a valve and control system that provides for improved pressure drop and flow rate characteristics as the valve is being stepped through incremental positions.
Further, when using an incremental system, a target pressure drop or valve position may fall between increments. Thus, if the valve positions are spaced apart so that a target position falls in between existing choke positions, the choke actuator may continually shift between adjacent positions in attempting to achieve the target flow conditions. Such a condition may occur between, for example, the 20% and 40% incremental positions wherein the change in flow rate and pressure drop is relatively large. Continual shifting may tend to wear the downhole components without providing the desired flow. Consequently, there is a need for a valve and control system adapted to provide the incremental positions necessary to avoid continual shifting and to meet the most likely flow needs for the valve.
Another limitation associated with some conventional valve actuators is that indexing mechanisms to step the actuators through incremental positions do not provide precise control. Thus, for example, when an actuator is indexing from a first incremental position to a second incremental position, the actuator may actually cause the valve to temporarily open past the second incremental position due to the design of the indexing mechanism. As a result, a surge in fluid flow may be caused by the temporary overshoot of the actuator. Such a fluid surge may damage the surrounding formation or cause the production of contaminants such as sand. Thus, a need exists for a valve actuator having more precise control than available with conventional valve actuators.